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MTU on VLAN interface

spamdrop
Occasional Visitor

MTU on VLAN interface

Hi,



I'm trying to enable jumbo frames on my VLANs, on a 5500G-EI switch. The switch is configured with VLAN interfaces, and acts as a router between the VLANs.



After enabling jumbo frames both on the relevant ports of the switch and on the end devices, everything is fine within the boundaries of a given VLAN.



However, inter-VLAN communication doesn't work anymore.



As far as I understand, this is because the VLAN interfaces of the switch have an MTU of 1500, which may prevent jumbo frames from crossing VLANs.





- Is the diagnosis correct?

- How do I set the MTU or enable jumbo frames on the VLAN interfaces?





Thanks!



SD
2 REPLIES
Luckycharms
Frequent Advisor

Re: MTU on VLAN interface

Your problem is that the "DF" bit is probably set for the Jumbo frames by the sending device.



If "DF" is set then your right you wont be able to Route these frames out Routed links/interfaces that dont have the same Frame Size.



If DF is not used then the Jumbo Frames will still function as normal within the same VLAN and should work fine across VLAN/Subnets and/or WAN interfaces where the MTU is smaller.



spamdrop
Occasional Visitor

Re: MTU on VLAN interface

Thanks for the information!



I hope it's possible to have a full clear path for jumbo frames all the way accross my VLANs, without fragmentation. We have a file server in a dedicated VLAN, and clients should be able to send jumbo frames to that filer without fragmentation!!



If its not possible, that's what I would call an annoying limitation. We also have a linux router, and increasing the MTU on all interfaces works perfectly even with packets that are routed from one interface to the other..